Various methods and devices have been used for transferring magnetic fields between different volumes. Magnetic field transfer devices may be utilized in inductive energy storage systems and in magnetic refrigeration devices. To accomplish a transfer of flux from one volume to another in a reversible and (substantially) lossless manner, several basic principles must be considered. First, the flux in a closed circuit should remain unchanged during any transient process. Second, according to the principle of least action, the difference between two complementary forms of energy integrated over the duration of the dynamic process should be minimized. If one of the energies is magnetic, the other energy may be kinetic or potential mechanical energy, electromechanical energy, and so forth. In electrodynamics, magnetic energy and electrostatic energy are complementary. Third, to ensure reversibility of the transfer process, there should be substantially no entropy production.
Capacitors often are connected in parallel with magnetic coils for use as a transfer element during inductive transfer between the coils. However, such a transfer element generally is required to accommodate about half the initial energy of the first coil in a complementary form during the transfer, in accord with the principle of least action. The initial energy is stored in the magnetic field of the coil and the complementary energy is stored in the electric field of the capacitor. Capacitors are quite limited in the amount of energy which they are able to store, and therefore may not be suitable for use as a transfer element in many applications.
Various methods for transferring magnetic fields are discussed in S. L. Wipf, "Reversible Energy Transfer Between Inductances", in Energy Storage, Compression and Switching, (book), Plenum Publishing Company, New York, N.Y., 1976, pp. 469-475. One system described in this article has a liquid metal homopolar transfer element in which a magnetohydrodynamic (MHD) medium flows at right angles to a magnetic field and to the current flow. The missing magnetic energy during transfer is converted into the kinetic energy of the liquid metal. Thus kinetic energy is employed to store the magnetic energy difference in a complementary form during transfer; the device therefore acts in the same way as a capacitor. It has an effective capacity proportional to the density of the medium and to 1/B.sup.2. A second system described achieves magnetic field transfer by the rotation of a shorted inductance coil magnetically coupled to a coupling coil which is part of a load circuit with at least one other coil. From the initial state of maximum coupling the movable shorted inductance coil is rotated and thus its coupling is reduced. The reduction of the coupling causes a transfer of energy from one coil to the other coil of the load circuit and subjects the rotating coil to an accelerating torque. When the coupling is zero the kinetic energy of the rotating coil is at its maximum; further rotation increases the coupling in the negative direction, decelerating the rotating coil. The transfer is completed and the rotating coil comes to rest when the coupling is again at maximum but in the opposite direction.
These transfer devices, where kinetic energy is used, instead of the electrostatic energy of a capacitor, can be much more compact than equivalent capacities. However, the containment of the mechanical forces, and especially the accelerating and decelerating forces, can pose uncommon design problems.
As a special case it is possible to make rotating inductive transfer elements where the sum of the magnetic energy stored in all the inductances is constant during the transfer. There is no necessity to be able to store kinetic energy during transfer.
Such a device is described in P. F. Smith, "Synchrotron Power Supplies Using Superconductive Energy Storage", Proceedings of the Second International Conference on Magnet Technology, 1967, pp. 589-593. The disadvantage is that the transfer element must be capable of storing, inductively, twice as much energy as the inductances between which the magnetic field is transferred. Smith also describes another device which is equivalent to a mechanically coupled motor and dynamo connected to an energy storage coil magnet and a synchrotron coil magnet.